# dBu Threshold

Over the past 10 years of my research and study of compressors, limiters and gates I do not remember any of the thresholds being in dBu units. If I did, they were too expensive to justify the purchase of one so I did not even waste my time on them.

Now I have come across some plate amplifiers with the DSP option that are using dBu instead of dB for all of their threshold parameters.

The 80% of the DSP function use dB which is the ratio of the speaker voltage divided by the amplifier voltage within the 20*Log( ) function.

dB Threshold = 20*Log(speaker voltage / amplifier voltage )

The calculation for this voltage is derived from the square root function of the wattage divided by the impedance.

Voltage = SQRT(watts rating * Load in ohms)

Now, dBu using a reference of .775 for the denominator for the equation 20*Log(Voltage/.775) and I could

To derive the dBu threshold setting for the DSP, it will require to find the dBu value of that voltage for each of the speaker and the amplifier. Then the difference between the speaker dBu minus the amplifier dBu shall be the threshold setting for the DSP function.

Now let’s walk through an example:

The speaker’s program power is 800 watts at 8 Ohm load = 80 volts

Now using the 20*Log(Voltage/.775) for dBu, dBu=20*Log(80/.775)=40.28 dBu

Now the amplifier power rating is 1000 watts at 8 Ohms load = 89 volts

One additional requirement for the amplifier is the input sensitivity specification. In this case = .775

Now using the 20*Log(Voltage/Amplifier Sensitivity) for dBu, the dBu=20*Log(89/.775)=41.24 dBu

In this case the threshold setting shall be dBu=speaker dBu – Amplifier dBu or 40.28-41.24= -0.96 dBu

Now let me put a twist on this. I was reviewing some the specifications of these plate amplifiers and I noticed that several of that had a sensitivity of 1.4. Looking further, I found three that had a sensitivity of 1.7.

Let’s use each of these specifications and run through the calculations again.

Sensitivity = 1.4

Now using the 20*Log(Voltage/Amplifier Sensitivity) for dBu, the dBu=20*Log(89/1.4)=36.11 dBu

In this case the threshold setting shall be dBu=speaker dBu – Amplifier dBu or 40.28-36.11= 4.17 dBu

Sensitivity = 1.7

Now using the 20*Log(Voltage/Amplifier Sensitivity) for dBu, the dBu=20*Log(89/1.7)=34.42 dBu

In this case the threshold setting shall be dBu=speaker dBu – Amplifier dBu or 40.28-34.42 = 5.86 dBu

Summarizing

• Amplifier sensitivity of .775 has a 1:1 ratio with the dB calculation
• Amplifier sensitivity of 1.4 has a 1:5.14 ratio with the dB calculation
• Amplifier sensitivity of 1.7 has a 1:6.83 ratio with the dB calculation

So the dBu calculation is taking in account of this input sensitivity during these calculations. I am still thinking about this because all I am concerned about is the output voltage and not how much voltage is required to trigger the operation so the amplifier.